©NationalInstruments Corporation B-1 SerialHardware and Software for Windows
B

Serial Port Information

This appendix describes the RS-232, RS-422, and RS-485 standards and
explainssome of the issues involved with these types of serial
communication.
TableA-1 lists the features of the RS-232, RS-422, and RS-485 standards.

RS-232

As specifiedin the ANSI/EIA-232-D Standard, Interface Between Data
TerminalEquipment and Data Circuit-Term inating Equipment Employing
SerialBi nary Data Interchange, RS-232 standardizes serial
communication betweencomputers, and between computer terminals and
modems. Most applications use the RS-232 standard to interface
peripheralsto personal computers. RS-232 uses transmission lines in which
the state of each signal is represented byreferen cing the voltagelevel of a
singleline to ground. RS-232 was designed for serial communication up to
TableB-1. RS-232,RS-422, and RS-485 Features
Feature RS-232 RS-422 RS-485
Typeof
transmission lines
Single ended Differential Differential
Maximum number
of drivers
1132
Maximum number
of receivers
110 32
Maximum cable
length
50ft 4,000ft 4,000ft
Maximumdata rate 20kbytes/s 10Mbytes/s 10Mbytes/s
MaximumCMV ±25V ±7V +12to –7V
Driveroutput 5to25V 2to6V 1.5to 6 V
Driverload >3kΩ100Ω60 Ω